Postprint version. Published in Biophysical Journal, Volume 101, Issue 4, August 17, 2011, pages 916-924.
The negatively charged proteoglycans (PG) provide compressive resistance to articular cartilage by means of their fixed charge density (FCD) and high osmotic pressure (πPG), and the collagen network (CN) provides the restraining forces to counterbalance πPG. Our objectives in this work were to: 1), account for collagen intrafibrillar water when transforming biochemical measurements into a FCD-πPG relationship; 2), compute πPG and CN contributions to the compressive behavior of full-thickness cartilage during bovine growth (fetal, calf, and adult) and human adult aging (young and old); and 3), predict the effect of depth from the articular surface on πPG in human aging. Extrafibrillar FCD (FCDEF) and πPG increased with bovine growth due to an increase in CN concentration, whereas PG concentration was steady. This maturation-related increase was amplified by compression. With normal human aging, FCDEF and πPG decreased. The πPG-values were close to equilibrium stress (σEQ) in all bovine and young human cartilage, but were only approximately half of σEQ in old human cartilage. Depthrelated variations in the strain, FCDEF, πPG, and CN stress profiles in human cartilage suggested a functional deterioration of the superficial layer with aging. These results suggest the utility of the FCD-πPG relationship for elucidating the contribution of matrix macromolecules to the biomechanical properties of cartilage.